This invention relates to cathodic protection of metallic structures such as the casings of oil, water and gas wells at large distances below the well head.
The process of corrosion of a metallic structure is essentially an electrolytic process involving the loss of electrons from the structure, for which an electrolyte is necessary. In the case of a metallic structure within the ground, such as the casing of an oil, water or gas well, the moist earth and/or subterranean water pockets act as the electrolyte. It has been found that without corrosion protection, these casings corrode and develop cracks and leaks.
One type of conventional corrosion protection involves putting a protective external coating on the casing. This method is available only for new wells.
However, it has been found that the cathodic elements of a metallic structure corrode less than the anodic elements. Therefore, another conventional method of corrosion protection in this environment is to attach a cathodic protection cable to the well head, at the surface, to supply current to the wellhead and thereby seek to render the entire metallic structure cathodic, i.e. negatively charged with respect to the surrounding earth. While this method works well for metallic portions of the structure at the surface, it has been found to be ineffective for those portions of the well structure at significant distances below the well head. This is so even when the amount of current is substantially increased or even doubled.
It is therefore an object of the present invention to provide corrosion protection for metallic structures at significant distances below the earth""s surface that avoids the above-described difficulties of the prior art.
It is a more specific object of the present invention to provide effective corrosion protection for well casings at significant distances below the well head.
It is a further object of the present invention to provide effective cathodic corrosion protection for well casings at significant distances below the well head.
It is another object of the present invention to provide cathodic corrosion protection that is safe to use for well casings at significant distances below the well head.
The above and other objects are achieved by the present invention which, in one embodiment, is directed to a downhole cathodic protection cable system for providing cathodic protection to a metallic structure below the earth""s surface. The system comprises an electrical connection structure approximately at the earth""s surface, an attachment shoe electrically connected to the metallic structure at a distance substantially below the earth""s surface, and an electrical cable having first and second ends, the first end being connected to the connection structure and the second end being electrically connected to the attachment shoe. The first end of the cable is electrically connected through the connection structure to a current source for providing a current to the cable sufficient to prevent substantial corrosion of a portion of the metallic structure surrounding the attachment shoe.
In accordance with an advantageous aspect of the present invention, the distance of the attachment shoe below the earth""s surface is greater than a distance at which a current supplied to the metallic structure at the earth""s surface can effectively prevent substantial corrosion, for example on the order of thousands of feet.
In a preferred embodiment, the attachment shoe provides a sturdy mechanical attachment of the second end of the cable to the metallic structure.
In a further preferred embodiment, the metallic structure includes the inner casing and outer casing of a well, the attachment shoe is connected to the inner casing, and the cable runs between the inner and outer casings from the attachment shoe up to a point substantially at the earth""s surface.
The downhole cathodic protection cable in accordance with the present invention provides cathodic protection to the deeper portions of the casing that cannot be protected using the conventional cathodic protection surface connection. It can be used in new wells and in existing wells by running the cable behind the well production tubing and then connecting it to the existing casing.
Moreover, the downhole cathodic protection cable in accordance with the present invention also provides cathodic protection above as well as below the point where the cable is connected to the casing.
A primary benefit of the downhole cathodic protection cable in accordance with the present invention is that it can prevent or minimize the occurrence of casing leaks, which can cost hundreds of thousands of dollars for repairs each year, as well as losses in oil production or water injection These and other objects, features and advantages of the present invention will be apparent from the following detailed description of the preferred embodiments taken in conjunction with the following drawings, wherein like reference numerals denote like elements.